The Polymerase Chain Reaction

When it comes to replicating DNA, PCR is the gold standard. Be it for cancer research, crime scene investigations, or paternity tests- they all use PCR to create enough identical strands of DNA for use from a single swab of cells!

So how does it work? Well we all know DNA has two strands, but what you might not know is that to replicate, those strands must be separated by heating, and used as templates for the addition of bases. Now, adding bases can only occur in one direction- on the 3′ end. With the help of the polymerase, bases (or nucleotides) can be attached, lengthening the DNA sequence. Of course, they aren’t added randomly- this is where the second strand of DNA comes in! The polymerase reads the template strand, and incorporates the partner base (A-T /C-G) onto the other strand. This happens again and again, cycling temperatures through hot (strand separation) to cold (base addition).

If we’re looking at a single gene on the strand of DNA, we won’t want the whole lot to be replicated- we just want that one gene millions of times. This is where we use a primer. A primer is a very short sequence (about 20 bases) which binds specifically to the DNA either side of the gene we want to amplify. This starts off the replication at that specific point, making the polymerase copy the gene in question more than any of the other parts of the DNA.

Each DNA strand will create 2 DNA helixes after being replicated, each of those will produce 2 more, giving 4 total. This goes on and on and on until you have millions (usually around 60 cycles- that’s nearly 600,000,000,000,000,000 times the original number of DNA strands!). Now there’s plenty of DNA use!

~By James Anderson